Field of the Invention
The invention relates to a method for coating the surface of inorganic particles, particularly of titanium dioxide pigment particles, with a dense skin of silicon dioxide and at least one further inorganic compound, particularly with aluminium oxide.
Technological Background of the Invention
Inorganic particles, particularly inorganic pigment particles, are often surface-treated in order to modify certain properties, such as surface charge, dispersing properties, acid resistance or light-fastness. Because of its high refractive index, titanium dioxide, in particular, is a high-quality pigment that is used in many sectors, e.g. in coatings, plastics, or fibres and paper. However, titanium dioxide is photoactive, meaning that free radicals are formed on the surface via electron-hole pairs as a result of exposure to ultraviolet radiation. These free radicals can enter into reactions with the substances present in the surrounding matrix, this possibly leading to destruction of the matrix.
One customary way of reducing the photoactivity of the titanium dioxide particles, i.e. of increasing their photochemical stability, is to cover the particles with the densest possible amorphous layer of silicon dioxide, a so-called “dense skin”, which is intended to prevent the formation of free radicals on the surface. It is, however, likewise known that the dense SiO2 skin impairs the tinting strength (TS), gloss and dispersibility of the pigment particles. For this reason, the pigment particles are customarily treated with aluminium oxide following application of the dense SiO2 skin.
Furthermore, various methods are known from the prior art for further improving surface treatment with a dense silicon dioxide skin and aluminium oxide. For example, DE 1 467 492 A discloses a method for improving both the tinting strength, the gloss and also the photochemical stability of titanium oxide, where the pigment particles are surface-coated twice with SiO2 and Al2O3 and finally subjected to heat treatment at 700° C.
It is generally assumed that the improvement in the tinting strength, gloss and dispersibility of pigments given a dense SiO2 skin and subsequent Al2O3 treatment is caused by changed surface properties—zeta potential and/or isoelectric point (IEP). It is generally known that the isoelectric point of aluminium oxide surfaces lies at a pH value of roughly 9, in contrast to silicon dioxide surfaces, whose isoelectric point lies at a pH value of roughly 2. However, surface analyses show that the known methods for sequential precipitation of an inner, dense SiO2 layer and an outer Al2O3 layer do not produce clearly separate layers. Rather, Al2O3 is incorporated into the SiO2 layer, such that a mixed layer of SiO2 and Al2O3 forms. This finding is backed by the results of measurements of the zeta potential and the isoelectric point of the particles. The isoelectric point of pigment surfaces provided with a dense SiO2 skin and subsequently treated with Al2O3, usually lies at a pH value of well below 9.
There is thus a need for a method with the help of which separate layers of inorganic compounds can be applied to the surface of inorganic particles having a dense SiO2 skin.